1. Field of the Invention
The present invention relates to a base station and a communication method.
2. Description of the Related Art
A communication system using a radio wave with a certain frequency band and a communication system using a radio wave with a frequency band close to that of the communication system before generate interference waves toward each other's frequency bands. As a result, interference between the communication systems occurs. For example, unnecessary waves such as leakage power and spurious power of one communication system become the interference waves for the other communication system.
As a method to avoid the interference between the communication systems, the one, which provides frequency bands called a guard band between the frequency bands used by the respective communication systems, has been heretofore used. Besides this method in order to avoid the interference, a random assign by a carrier sense has been used. This is a method to avoid the interference between the communication systems in the following manner: Noise power values of frequencies are measured, and a frequency containing large noise power is judged that it is interfered from other communication systems. Then, a frequency with less noise power is used.
In addition, as to the interference wave such as the spurious power generated in a specific frequency, a method has been used, which avoids the interference by switching a time slot and changing timing of transmission/reception or by switching a frequency. Furthermore, a method has been used, in which a transmitter transmits signals, which are spreaded over a wide frequency band by spreading codes, and then a receiver spreades when the receiver demodulates the spreaded signals, thus removing the interference wave components.
However, in the conventional method to provide the guard band, when a frequency band except the frequency band used by the communication system is wide, the guard band provided between the frequency bands of the communication systems has needed to be made wide. Therefore, the guard band using a wide frequency band has been provided for the interference between the systems, which changes depending on locations of a base station and a terminal equipment, which constitute the communication system and does not occur all the time. As a result, the use efficiency of the frequency has been decreased. Furthermore, in the case of the random assign by the carrier sense, the frequency is not used, which is judged that it is interfered from other communication system for the reason that it shows large noise power in measuring the noise power. Accordingly, the use efficiency of the frequency has been decreased.
As a result of the decrease in the use efficiency of the frequency as described above, the base station constituting the communication system can not maintain satisfactorily the number of terminal equipments connecting lines with the base station, that is, the number of users accommodated in the communication system (hereinafter referred to as a “user capacity”), who use the communication system. Thus the base station has not been able to maintain communication quality.
Although the switching of the time slot and the switching of the frequency are effective for the interference wave such as the spurious power which is generated in the specific frequency, they can not deal with the interference wave such as the leakage power, which extends all over the frequencies. Furthermore, with a CDMA-TDD system using a narrow frequency band, frequency switching cannot be performed. For this reason, the method to avoid the interference wave by switching the frequency can not be applied to the CDMA-TDD system. Accordingly, the base station has not been able to avoid influences due to the interference from other communication systems even if the methods above were employed. Thus, the base station has not been able to maintain the user capacity satisfactorily, and maintain the communication quality.
On the other hand, the method to remove the interference wave component by the despreading is effective to the interference all over the used frequency band and continuous interference. Therefore, this method has an advantage that interference can be somewhat avoided. However, even with this method, when a certain quantity of interference or above is generated, the user capacity comes to be incapable of being maintained satisfactorily, and it makes it impossible to maintain communication quality.
Furthermore, even when the interference increases and the user capacity decreases, the terminal equipment located near the base station is easily connected to the base station, since the terminal equipment has leeway in transmission power. However, since a terminal equipment located at the edge of a wireless area does not have leeway in the transmission power, it becomes difficult to connect the terminal equipment to the base station, as the user capacity decreases due to an interference increase. Specifically, when the interference increases and the user capacity decreases, the terminal equipment located near the base station comes to connect a line with the base station more easily than the terminal equipment located at the edge of the wireless area covered with the base station. Therefore, terminal equipments connecting lines and communicating with the base station concentrate near the base station. Then, the user capacity at the edge of the wireless area covered by the base station is decreased. As a result, a density distribution of the terminal equipments connecting lines with the base station is more near the base station, and less at the edge of the wireless area.
As described above, it has been impossible to maintain a constant density distribution of the terminal equipments communicating with the base station. Therefore, it has been impossible to maintain the user capacity uniformly in the wireless area. Consequently, in an area where the communication system offers communication services, a situation turns disadvantageous to the terminal equipments located at the edge of the wireless area, and it has been difficult to offer the communication services equally to the plurality of terminal equipments.